The SI unit of viscosity is the pascal-second (Pa·s). It is derived from the formula for dynamic viscosity, μ, given by Newton's law of viscosity: $\tau =\mu \frac{du}{dy}$, where τ is shear stress (in pascals, Pa) and du/dy is the velocity gradient. Therefore, μ = τ / (du/dy) has units of Pa·s. The poise (P) is a non-SI unit from the CGS system, where 1 Pa·s = 10 P. The poiseuille is an older name for the Pa·s. The stoke is the CGS unit for kinematic viscosity.

Final Answer: poiseuille